Just a few yards from the choked M4 motorway, beyond the massive settling tanks and a steaming, 500-tonne mountain of black sewage sludge at Slough treatment works, a modern alchemy is taking place that could potentially keep the world in food for a few more years.

The plant is taking the tiny quantities of phosphorus contained in the poo of the Berkshire town’s 140,000 people and turning it into high-quality fertiliser fit to grow organic garden vegetables.

At one end of the novel process in Europe’s first “nutrient recovery reactor”, the human waste is dark and “earthy” smelling. At the other end, bright white, odourless phosphorus-rich pellets drop into sacks. The sewage workers euphemistically say they are “harvesting pearls”. Thames Water, which owns the facility, says it is making “Viagra for plants”.

According to the water company, Slough’s excreta has a “unique vintage”, and contains more phosphorus than any other area in south-east England, possibly because of the quantity of meat eaten in the town or because it boasts several large food processing and pharmacueutical works. The company expects to make £200,000 a year from the combination of selling 150 tonnes of its fertiliser to farmers and gardeners, and not having to spend as much money on chemicals to unblock pipes.

“We reckon using this technology Britain could save 20% of the 138,000 tonnes of phosphorus fertiliser that it imports a year,” says Piers Clark, Thames Water’s commercial director. “Phosphorus is a fast-depleting, non-renewable resource which we will run out of. Without it, all life on the planet will take a nosedive.”